Despite the continuous risk of harm their availability creates, hazardous gases are essential to many industries for use in a variety of manufacturing processes. For instance, the semiconductor industry utilizes certain hazardous gases for deposition and etching processes. Used during the fabrication of integrated circuits, the hazardous gases are processed in systems such as chemical vapor deposition and diffusion reactors.
This industry demand for hazardous gases invariably requires the storage of these gases at a location proximate to its ultimate use. In most cases the compressed gas is stored in high pressure metal cylinders, which are readily available at most bulk gas suppliers.
As one would expect, a danger is posed that any one of the cylinder containers may leak at some future date. The release of hazardous gases into the environment is an intolerable form of deadly air pollution. As a result, local standards have been promulgated to which the users of stored hazardous gases must adhere in order to prevent or certainly minimize such risks.
Specifically, Article 80 of the Uniform Fire Code requires that some type of treatment system be installed at the storage location for the isolation and treatment of potentially escaping gas. According to Article 80 requirements, an acceptable treatment process for the accidental release of the entire contents of the largest single container of hazardous gas in a five or thirty minute period, depending on the state (vapor or liquid) of the gas prior to escape, must reduce its effluent concentration to less than one-half of the "immediately dangerous to life and health value" (IDLH) set by the Occupational Safety and Health Administration. If the gas is stored in the form of vapor, the contents are considered releasable over a five minute period. For gas stored in the form of a condensed liquid, the appropriate time period for release is thirty minutes.
In addition to the treatment requirements, Article 80 requires that hazardous compressed gases be stored in a ventilated environment, wherein any escaping gas is isolated and directed to the treatment process. The conventional apparatus for furnishing a ventilated environment is generally referred to as a gas storage cabinet. This cabinet is plumbed into a ventilation system that applies a negative pressure to the cabinet. As air is drawn through and out of the cabinet, any escaping gas is simultaneously drawn from the cabinet as a hazardous gas mixture.
Certain provisions of the Uniform Fire Code require that any access part in a ventilating gas cabinet have a minimum face velocity of 200 ft/min of air to prevent back diffusion into the work area. Typically, this level translates into 250 to 350 cubic feet per minute (CFM) of air flow from any one gas cabinet.
Where hazardous gas is used in the processing of other products, the processed gas is removed as "spent" gas. The spent gas is exhausted from the manufacturing process in the form of waste effluent which may be recycled. However, most often this gaseous effluent cannot be reused and must therefore be treated and subsequently released.
In general industry practices, on-going ventilation and treatment is provided for the spent hazardous effluent gas. However, because the flow of hazardous gas through the manufacturing process is generally at low rates, treatment of the hazardous effluent gas is often accomplished inexpensively.
In the case of chemical vapor deposition reactors, referred to above, hazardous effluent gas flow rates are in the range of 1/2 to 11/2 CFM per reactor, permitting effective and economical treatment by relatively low volumetric flow combustion devices, i.e., low volume incinerators. Low volume incinerators are effective at incinerating the hazardous gas into a non-hazardous waste product. An accidental release of hazardous gas from a supply cylinder within the processing flow system can adequately be addressed by the low volume incinerator. However, an accidental release into the ventilation system involves too excessive an air flow for such low volume incinerators. Consequently, safety and capacity requirements dictate using a larger, more expensive incinerator to insure against the risk of such an event. However, in addition to the considerable capital expense of the larger incinerator, operating expenses would significantly increase as it is necessary to operate the incinerator continuously to guard against an accidental release.
The air withdrawn from the gas cabinets is typically processed by a water scrubber. Many hazardous gases, however, are not water soluble and are not efficiently removed by water scrubbers. For example, conventional scrubbers are not effective to remove hydride gases such as arsine, diborane, and phosphine. A water scrubber is a device wherein water is injected at a direction generally countercurrent to a flow of gas to be treated. The gas is dissolved and the solution settles into a collection reservoir where it may be directed for further treatment.
For material released into the ventilation system that is not water soluble, no presently-available means exist for efficiently and economically treating such materials to acceptable levels. However, there are methods presently available for chemically treating such materials. For instance, liquid scrubbers using compounds such KMnO4, KOH and other similar compounds, have been tested and shown to improve the scrubbing efficiency insoluble hazardous gas so as to permit some level of treatment. However, the results of such tests indicate average capabilities of only 50% treatment with each scrubber. This less-than-efficient rate of treatment often proves to be uneconomical as multiple scrubbers in series are required in order to treat accidentally released gas.
By way of illustration, the following hypothetical projection is offered. There is stored a 4-pound bottle of 100% arsine (AsH3) as the largest container in a group of six gas cabinets containing similar materials, each cabinet furnished with 250 CFM of ventilation. If the exhaust from each gas storage cabinet is manifolded together as part of a unitary treatment system, the total air flow through the treatment device is 1500 CFM. It must be noted that AsH3 stored in high pressure containers is in a liquid state. Therefore the following calculations are based on the release of 4 pounds of AsH3 over a 30 minute isolation and treatment period as permitted by Article 80 of the Uniform Fire Code.
Assume for the moment that this bottle of Arsine develops a leak into the air moving through its respective gas cabinet. The resulting permissible flow rate for 4 pounds of escaping Arsine in a 30 minute period is 0.13 lbs/minute or 60.4 grams/minute. Based on a molecular weight of 76.0 grams for AsH3, the required treatment rate is 60.4/76.0=0.8 mole/minute. At 22.25 liters/mole of 100% AsH3, the release rate translates to 17.8 liters/minute or 0.64 CFM. The effluent concentration of Arsine gas is therefore 0.64/1500=424 parts per million (ppm). The IDLH of 100% AsH3 established by OSHA is 6 ppm. Pursuant to Article 80, the level of required treatment is one-half of the IDLH, or 3 ppm, in order to achieve compliance. Beginning with a release concentration of 424 ppm, seven 50%-efficient liquid scrubbers in series, each large enough to handle a 1500 cfm airflow, would be required to achieve this result. The economic impracticability of such an arrangement is readily apparent.
There is, therefore, a need for a system to efficiently treat a gas stream containing such hazardous gases to reduce their concentration to acceptable levels. In addition, there is a need for such an efficient treatment means that is also cost-effective in treating accidentally-released hazardous gas from bulk storage containers.